To build a team, first you should take a look at the Leader Skill portion of the cards (I prefer to use "card" as opposed to "monster"). Generally, the more ATK multiplier a card has in Leader Skill, the stronger it is to use as a leader, but it's not always the case because now we have "only" 25x ATK multiplier cards but very strong because of other factors built into the Leader Skill.

For example, let's take a look at your Dark cards, the ones you have more than any other attributes.

She has 8x ATK multiplier by her own. If you use the same card as friend helper, you got 64x ATK multiplier. Pretty strong for early game and can clear all Normals and some Technicals dungeons and even some Mythical or Legend Plus difficulty of Special Dungeons. She needs you to make a five-orbs match, one of them being enhanced (orbs with +) while making a Fire, Water and Dark match. You'd want to make a mono dark team to get a good damage across the team, and having one or two Dark Orb Enhance awakenings helps in damage.

Haku.
In her Reincarnated form, she is good because she has passive 51% damage shield that will always on all the time but only have 20.25x ATK multiplier with double Haku leader, but she has double TPA and 7c awakenings and requires you to match two of the following orbs: Fire, Dark or Water attributes. If you can make a TPA and pulled 7+ combo, she can crank up pretty good damage.

The hardest part is pulling a TPA while making 7+ combo because of the lack of Time Extend (TE) awakenings across the team but if you fast enough you could pulled it off.

Right now, your focus should be to develop your current team to the max, that is all cards max level, max skill and fully plussed (+297).

You can build other team to your liking but the principal stays the same: Stick to your current team, develop it to max to take more harder contents while gathering more cards, either farmable or through REM.